This invention is directed to fibers, and more particularly to methods of making fibers from .tbd.SiO-- containing materials by coextruding a removable organic polymer as an outer sheath, around an inner sheath formed of an extrudable and curable .tbd.SiO-- containing material.
Numerous types of elastomeric fibers are known in the art including fibers characterized by a glass transition temperature below room temperature coupled with high elongation at break, low modulus, and high degrees of recovery from deformation. One of the most important physical properties is the elastic power defined as the force encountered in stretching and retraction repeatedly and any hysteresis or set which remains. Although silicone elastomers are generally weaker than most organic elastomers because of their lower modulus and tensile strength, they can be formulated to have low hysteresis and high elastic power.
Two of the major polymers used in manufacturing elastomeric fibers are natural and synthetic rubber, and various polyurethane based polymeric materials. Such materials, however, suffer from numerous disadvantages. The main deficiency of many of these elastomeric fibers is stretch induced crystallization. While this problem is observed primarily with fibers based on natural rubber, it is also observed to some degree in synthetic elastomers such as spandex polyurethane based fibers. This crystallization occurs as sufficient orientation of the rubber chains takes place at high elongation, and leads to dramatic changes in the mechanical profile of the fiber on subsequent stretching. These changes include an increase in the modulus and lower elongation at break, which are critical for fibers to survive textile processing and wear without breakage.
Another problem of prior art fibers is oxidative degradation which is caused by heat, light, atmospheric fumes, chemical agents, or ultraviolet (UV) radiation. The degradation of the fibers by such agents alters the structure of the polymer, and can drastically affect its mechanical properties. Most notably, chlorine is known to degrade polymers by a free radical chain reaction mechanism. Spandex fibers based on polyether soft segments are particularly susceptible to oxidation and must be protected. While spandex fibers based on polyester soft segments are not as susceptible to oxidation, they tend to hydrolyze at low or high pH values. Other problems with elastomeric fibers for the textile industry relate to their discoloration and staining.
The present invention seeks to overcome these disadvantages, and uses a fiber made from an .tbd.SiO-- containing material which has a protective outer sheath that supports the fiber during manufacture. While European Patent Application 378194 (Jul. 18, 1990) describes an elastomeric fiber spun with a protective sheath, it relates to a polyurethane core arranged in the center of a polyamide sheath, yielding a composite filament yarn. However, unlike our invention, the core in EP 378194 is not an .tbd.SiO-- containing material, and the protective sheath is not removed but forms an integral part of the composite yarn.